Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes an electrode lead implanted at the desired stimulation site and an implantable pulse generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the electrode lead or indirectly to the electrode lead via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, rate, and burst rate of the stimulation pulses.
The neurostimulation system may further comprise a handheld remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses). The circuitry of the neurostimulator may also include voltage converters, power regulators, output coupling capacitors, and other elements as needed to produce constant voltage or constant current stimulus pulses.
Single source current regulated and voltage regulated neurostimulators are highly limited in their ability to shape the current distribution and electric field around the electrode array used to activate excitable tissue. In essence, the electric field is determined by the electrode array geometry and the impedance profile of the surrounding tissue.
Multiple independent current source neurostimulators were developed to address this limitation. These neurostimulator types can be used to more precisely control the current distribution in tissue, and thus more selectively activate excitable tissue. This augments the capability inherent in the electrode array geometry and limits the influence of the surrounding impedance profile. In one technique particularly useful for SCS, three electrodes are rostro-caudally arranged along the spinal cord of the patient, with the center electrode configured as a cathode, and the top and bottom flanking electrodes configured as anodes, thereby focusing the stimulation energy at the spinal cord tissue adjacent the center electrode. The shape of the electric field produced by multiple independent current source neurostimulators, however, is still limited to what can be achieved by superposition of current sources in a conductive medium. In addition, current sources are less capable of controlling the electric field potential, which is determined by tissue impedance. Multiple independent voltage source neurostimulators, in principle, can be used to more precisely control the electric field; however, the currents delivered by the voltage sources change with impedance.
Conventional battery-operated neurostimulators typically apply stimulation pulses to the tissue that are referenced to an internal circuit voltage in the neurostimulator, with a relatively low impedance connection being located between one or more stimulation electrodes and internal circuitry. This relatively low impedance effectively clamps the voltage on these stimulation electrodes to the internal circuit voltage, as described in U.S. patent application Ser. No. 12/821,043, entitled “Symmetrical Output Neurostimulation Device,” which is expressly incorporate herein by reference.”
Because the voltage at the unregulated side of the electrode will be clamped to the voltage of the internal circuitry, and because the stimulation output circuitry may be unbalanced in that some components in the circuitry (coupling capacitors, protection circuits, etc.) may be present on the cathode side of the circuit but not the anode side of the circuit, or vice versa, the output stimulation circuitry between the cathode and the anode will be asymmetrical, such that the cathode and the anode will be asymmetrically referenced to the internal circuit. For example, a shift in voltage in the output stimulation circuit results in asymmetrical voltage shifts between the anodes and cathodes, as described in U.S. patent application Ser. No. 12/821,043. The asymmetry between anodes and cathodes in the output stimulation circuitry may be associated with undesired side effects during stimulation that lead to reduced patient comfort. In particular, parasitic coupling of the common mode signal to the implantable device can give rise to an additional stimulation signal that is superimposed on the differential stimulation signal.
In addition to the problem of asymmetry in the output stimulation circuit, referencing the voltage at the cathodes and anodes to an internal circuit may require excessive voltage levels at the cathodes and anodes in order to maintain the desired voltage potential therebetween. For example, if the desired voltage potential between a cathode and an anode is 5V, and if the internal voltage is 20V, the voltage at the anode would have to be 25V and the voltage at the cathode would have to be 20V. The increased voltage at the electrodes will increase the voltage relative to the tissue, which may cause problems such as unwanted stimulation and even electro-chemical reactions resulting in corrosion of the electrodes.
There, thus, remains a need for an improved method and system for conveying stimulation to tissue in a controlled manner.